Open-cell particulate foams

ABSTRACT

Open-cell moldable foams are based on a thermoplastic matrix which comprises A. from 99 to 50% by weight of a styrene polymer and B. from 1 to 50% by weight of a polymer of low compatibility with the styrene polymer.

The invention relates to open-cell moldable foams based on styrenepolymers.

An advantage of open-cell foams is that they can be evacuated. Theresultant evacuated vacuum moldings have considerably lower thermalconductivity and are therefore significantly better insulators thanconventional foams.

Open-cell extruded foam sheets and open-cell extruded foam boards basedon styrene polymers are known, e.g. from EP-A 642,907, WO 9600258 and WO96/34038. They are produced by extruding a polystyrene melt to which avolatile blowing agent and a nucleating agent have been added, attemperatures of from 110 to 140° C. In WO 98/58991, from 0.1 to 7% byweight of an ethylene-vinyl acetate copolymer is to be added to thepolystyrene during the process.

However, foam sheets and foam boards are of simple shape. Theirthickness and width is predetermined by the dimensions of the extrusionequipment.

Polystyrene foam moldings of any desired dimensions and shape may beproduced in a known manner by foaming polystyrene pellets comprisingblowing agents and sintering the resultant moldable foam.

It is an object of the present invention to provide open-cell moldablefoams based on styrene polymers.

We have found that this object is achieved if the thermoplastic matrixcomprises A. from 99 to 50% by weight of a styrene polymer and B. from 1to 50% by weight of a polymer of low compatibility with A.

A are polystyrene and copolymers of styrene which incorporate at least80% by weight of styrene in the polymer. Examples of comonomers areα-methylstyrene, ring-halogenated styrenes, ring-alkylated styrenes,acrylonitrile, (meth)acrylates of alcohols having from 1 to 8 carbonatoms, N-vinyl compounds, such as vinylcarbazole, or else small amountsof compounds which have two polymerizable double bonds, for examplebutadiene, divinylbenzene or butanediol diacrylate. Polystyrene ispreferred.

The mixture of polymers A+B comprises from 1 to 50% by weight,preferably from 2 to 35% by weight, of a polymer B of low compatibilitywith A. The preferred amount is from 3 to 10% by weight if polymer B ispolypropylene, and for polymethyl methacrylate it is from 10 to 25% byweight.

The incompatibility of B with A is determined as follows:

The glass transition temperature of the mixture of polymers is measuredby DSC. If the glass transition points of the individual components aredetectable separately in the DSC curve and have not been markedlyshifted compared with the glass transition points of the individualcomponents when pure, incompatibility is present. In case of doubt, theincompatibility of A with B can also be determined by electronmicroscopy, which if incompatibility is present shows two distinctphases if appropriate contrasting is used, phase A being the coherentphase.

Suitable polymers B are polyethylene, polypropylene, ethylene-vinylacetate copolymer, polyurethane, polyamides, polycarbonate, polyethersulfones, polyoxymethylene, polyvinyl chloride and polyimide. Preferenceis given to polymethyl acrylate and polypropylene.

Another aspect of the present invention is that of open-cell moldablefoams with increased heat resistance. These are obtained if the mixtureof polymers A+B also comprises from 5 to 50% by weight, preferably from10 to 30% by weight, based on A+B, of a thermoplastic C with a glasstransition temperature above 140° C., preferably above 180° C. Examplesof suitable thermoplastics C are styrene-maleic anhydride copolymers andpolyphenylene sulfide, and preference is given to polyphenylene ethers.Adding the thermoplastic C raises the glass transition temperature ofthe thermoplastic matrix A+C to above 105° C., with the result that thefoam does not soften, e.g. when vacuum panels are foamed intopolyurethane.

The starting material for producing the novel open-cell moldable foamsis pellets comprising blowing agent. To prepare these, the polymers A, Band, if desired, C are first mixed in the melt, with conventionaladditives.

Conventional additives and/or auxiliaries which may be added to thethermoplastic matrix are conventional amounts of stabilizers, dyes,fillers, flame retardants and/or nucleating agents, preferably from 1 to10% by weight, based on A+B+C, of an infrared absorber, e.g. aluminumpowder or titanium dioxide powder, or of particulate carbon, inparticular graphite powder, further increasing the open-cell content.

The pellets obtained after mixing are then impregnated with volatileblowing agents, using conventional processes, preferably in aqueoussuspension at from 100 to 150° C. and at a pressure of from 3 to 15 bar.Suitable blowing agents are saturated aliphatic C₄-C₇ hydrocarbons, inparticular pentane.

In principle it is also possible for pellets A which comprise blowingagent to be mixed in the extruder with pellets B, and, whereappropriate, C; alternatively, blowing agent may be introduced duringthe mixing of A, B, and, where appropriate, C. In these processes, themelt extrudate from the extruder is then usefully subjected, while underpressure, to underwater die-face cutting for pelletization.

The pellets comprising blowing agents are then prefoamed, using steam attemperatures of from 60 to 160° C., whereupon—due to the polymer B whichhas been added and has low compatibility with polystyrene—the cellmembranes break open to give an open-cell moldable foam. The density ofthe moldable foam can be adjusted as desired by repeated foaming.Finally, the moldable foam is sintered, using steam or hot air, innon-gas-tight molds, to give foam moldings. According to the invention,these have an open-cell content of more than 80%, preferably more than90%, in particular at least 95%. If the open-cell content is too low themolding cannot be adequately evacuated. Its cells then hold too muchgas, and thermal conductivity is not lowered sufficiently.

The novel open-cell molded foams may be evacuated to give vacuummoldings. These are used as vacuum panels in low-temperature insulationsystems, e.g. in refrigerators, freezers and low-temperaturetransportation systems.

EXAMPLE 1 Inventive

A twin-screw extruder is used to prepare a blend made from 95% ofpolystyrene (VPT, BASF AG) and 5% of polypropylene (3200 MC, BASF AG).20 kg of the pellets obtained by cold cutting (maximum diameter about1.5 mm) are placed in a pressure-tight stirred vessel with an initialcharge of 20 kg of deionized water, 35 g of sodium pyrophosphate and 70g of magnesium sulfate (Epsom salts) and 1.8 g of K 30 emulsifier assuspension stabilizer. The contents of the vessel are heated to 125° C.,and once 110° C. has been reached 1600 g of pentane are metered in.After the vessel has been held at 125° C. for 4 h it is cooled to roomtemperature. This gives compact beads, which were foamed using steam ina batch pressure-prefoamer as known for processing EPS, at a gagepressure of 0.3 bar. The resultant foam beads had a bulk density of 15g/l. The individual beads were sintered to obtain a foam molding of thesame density. The resultant foam molding had an open-cell content of95%. The open-cell content was determined to ASTM D 2856-87, method C,using an Accupyc 1330 device.

EXAMPLE 2 Comparison

Similar to Example 1, but without polypropylene. The resultant foammolding had an open-cell content of 0%.

EXAMPLE 3 Inventive

Similar to Example 1, but with 80% of polystyrene and 20% of polymethylmethacrylate (Lucryl G77E). The resultant foam molding had an open-cellcontent of 95%.

We claim:
 1. Moldable foam pellets having an open-cell content of morethan 80%, wherein the thermoplastic matrix comprises A. from 99 to 50%by weight of a styrene polymer, and B. from 1 to 50% by weight of apolymer of low compatibility with the styrene polymer, and C. from 5 to50% by weight, based on A+B, of a thermoplastic which is compatible withA and has a glass transition temperature above 140° C.
 2. A moldablefoam as claimed in claim 1, wherein the thermoplastic A is polystyrene.3. A moldable foam as claimed in claim 1, wherein the polymer B ispolyethylene, polypropylene, an ethylene-vinyl acetate copolymer, apolyurethane or polymethyl methacrylate.
 4. The moldable foam pellets asclaimed in claim 1, wherein the thermoplastic C is a polyphenyleneether.
 5. A moldable foam as claimed in claim 1, which comprises from 1to 10% by weight, based on A+B+C, of athermanous particles, preferablygraphite.
 6. A process for producing the moldable foam pellets asclaimed in claim 1, by impregnation of pellets which comprise thepolymers A, B and C with from 3 to 15% by weight, based on A+B+C, of avolatile blowing agent in aqueous suspension under superatmosphericpressure and elevated temperature, and expansion of the pelletscomprising blowing agents, to give open-cell moldable foam pellets. 7.An open-cell foam molding prepared from the moldable foam pellets asclaimed in claim
 1. 8. The moldable foam pellets as claimed in claim 1,wherein the glass transition temperature of the thermoplastic C is above180° C.
 9. The moldable foam pellets as claimed in claim 1, wherein thethermoplastic C is selected from the group consisting of styrene-maleicanhydride copolymers, polyphenylene sulphide, and polyphenylene ethers.10. The moldable foam pellets as claimed in claim 1, wherein the styrenepolymer A is a copolymer.
 11. The moldable foam pellets as claimed inclaim 10, wherein the copolymer is a copolymer of styrene with a monomerselected from the group consisting of α-methylstyrene, ring-halogenatedstyrenes, ring-alkylated styrenes, acrylonitrile, (meth)acrylates ofalcohols having from 1 to 8 carbon atoms, N-vinyl compounds,vinylcarbazole, butadiene, divinylbenzene, and butanediol diacrylate.12. The moldable foam pellets as claimed in claim 1, wherein thethermoplastic matrix comprises 2 to 35% by weight of a polymer B basedon the amount of A+B.
 13. The moldable foam pellets as claimed in claim1, wherein the polymer B is polypropylene in an amount of from 3 to 10%by weight based on the amount of A+B.
 14. The moldable foam pellets asclaimed in claim 1, wherein the polymer B is polymethyl methacrylate inan amount of 10 to 25% by weight based on the amount of A+B.
 15. Themoldable foam pellets as claimed in claim 1, wherein the amount ofthermoplastic C is 10 to 30% by weight based on the amount of A+B. 16.The moldable foam pellets as claimed in claim 1, further comprisingadditional additives and/or auxiliaries.
 17. The moldable foam pelletsas claimed in claim 16, wherein the additional additives and/orauxiliaries are selected from the group consisting of stabilizers, dyes,fillers, flame retardants, nucleating agents, infrared absorbers,aluminum powder, titanium dioxide powder, particulate carbon,particulate graphite and mixtures thereof.
 18. The moldable foam pelletsas claimed in claim 16, wherein the additional additives and/orauxiliaries are present in an amount of 1 to 10% by weight based on theamount of A+B+C.
 19. The moldable foam pellets as claimed in claim 1,further comprising a blowing agent.